Many implantable medical devices, such as cardiac pacemakers and the like, are generally designed to be implanted in a “pocket” of fatty tissue near the patient's upper chest or lower abdomen. Accordingly, electronic circuits within the device are hermetically sealed within a housing made of a material that is compatible with body tissue, such as titanium or stainless steel. Electrical connection from the outside with the electronic circuits within the hermetically sealed housing is accomplished via a connector assembly, often referred to as a header or header assembly that is mounted on the housing. The header is generally constructed of a polymeric material, such as a polyurethane or the like. Feedthrough terminals that pass through the hermetically sealed housing are connected with the pacemaker electronic circuits in the housing and with a lead pin receptacle in the connector assembly. At a proximal end, a lead conductor is provided with a lead pin for being received in the connector receptacle and at a distal end, the lead conductor is provided with electrodes for electrical tissue stimulation at the desired tissue location.
In order to ensure good electrical contact between the circuits with the housing and the outside, and to provide sufficient strength of the implantable device, it is desirable to achieve a strong and reliable joint or bond between the polymeric header and the metallic housing. Generally, the header is connected to the housing by either injecting a thermosetting polymer (such as an epoxy, for example) into the header-housing interface through a backfill process or the header-housing is injection molded. This creates the desired attachment and electrical sealing. In either case, the polymer utilized must bond to the housing in order to attach the header properly. In many cases, an anchor is also welded to the housing and used to further strengthen the bond.
Generally, in order for the polymer to effectively bond with the housing, the bonding surface on the housing is roughened to increase the surface area that is subsequently bonded to thereby decrease the shear stress on the interface created between the housing and header. Surface roughening has conventionally been accomplished using a grit blasting process wherein the bonding surface is blasted with silicone particles using compressed air. The silicone particles become embedded in the surface and create a textured surface to which the polymer bonds. Although conventional grit blasting processes suitably increase the surface area of the treated surface to improve the resulting bond, it would be desirable to provide alternative improved processes for increasing the surface area of the bonding surface and improving the resulting bonds between a metallic housing and a polymeric header.